This invention relates to a process control system which controls a process by the combination of feed forward control and feedback control, and more particularly to a process control system suitable for the adaptive control of a process whose dynamic characteristics are variable depending on the factors such as secular variations.
A process control method is well known in which a feed forward control signal is determined as a function of a demand for a specific process and is modified on the basis of a process feedback signal applied from a feedback loop of a controlled variable for determining by calculation the desired value of the control variable of the process. Such a control method is commonly employed for the control of various processes.
Such a control method is disclosed in, for example, U.S. Pat. No. 3,894,396. In this U.S. patent, the desired values of flow rates of feed water, fuel, air and recirculation gases supplied to a boiler in a heat power plant are arithmetically calculated according to individual predetermined rates so that they meet the plant load demand. A feed forward control signal is produced on the basis of these calculated flow rate values to be used for the control of the opening of a feed water control valve, a fuel control valve, an air flow control damper and a gas recirculation flow control damper which control actually the flow rates of feed water, fuel, air and recirculation gases supplied to the boiler. The feed forward control signal is modified by individual feed forward modifying signals obtained by arithmetic calculations based on a main steam pressure error, a main steam temperature error, an O.sub.2 error and a gas recirculation flow error, so that the modified feed forward control signal can be used to control the opening of the feed water control valve, fuel control valve, air flow control damper and gas recirculation flow control damper. The rates used for the determination of the feed forward control signal are calculated according to algebraic functions which are so pre-selected that the main steam pressure error, main steam temperature error, O.sub.2 error and gas recirculation flow error are zero in the steady state of the process. Therefore, when these functions are always appropriate, the feedback correction signals act to absorb solely transient variations in the controlled variables so that the entire plant can be controlled to operate with a quick response.
However, the performance of such a power plant varies gradually due to deposit of soot in the boiler, contamination of pipe inner walls and other secular variations. Thus, even when the functions used for the arithmetic calculation of the feed forward control signal were originally properly set, these functions would become unfit for the purpose of control of the controlled variables after an extended period of continuous operation, and the resultant errors would be absorbed in the feedback correction signals. Since the feedback control system would not respond until an error appears in the parameter of one or more of the controlled variables, the process control with the quick response as in the initial stage of plant operation would become impossible in such a situation.
In order to prevent the undesirable reduction in the response of the process control system due to such secular variations of the characteristics of the controlled variables, it has been a common practice to periodically re-set the functions used for the determination of the feed forward control signal. However, this re-setting has required a great deal of costs and labors.